Nanostructures are building blocks for next generation materials or devices. A nanostructure may exhibit any configuration, including, but not limited to, a rod-like structure such as a nanowire or a tube-like structure such as a nanotube, among many other possibilities. Nanostructures offer a vast array of unique and technologically useful properties, which depend on their atomic- and nanometer-scale structure (e.g., diameter, length, morphology, crystal structure, composition, and heterostructure). Bottom-up growth permits many structures that are inaccessible in the bulk or thin films.
Existing methods for producing high quality nanostructures have many flaws. By way of example, known bottom-up vapor-liquid-solid (VLS) synthesis, while currently offering the best control of semiconductor nanowire structure, occurs on flat substrates. Such 2-D processes (i.e., with areal scaling) cannot achieve high productivities and, therefore, cannot offer economically competitive scale-up. 3-D processes (i.e., with volumetric scaling) would be ideal for applications requiring large quantities of materials such as thermoelectrics, photovoltaics and large-area electronics. However, the known candidates, including solution-liquid-solid (SLS) synthesis or aerotaxy, suffer from uncontrolled agglomeration and/or inadequate control of nanowire structure.
International Publication No. WO/2013114218 entitled “High-throughput continuous gas-phase synthesis of nanowires with tunable properties” pertains to “seed particles suspended in a gas” as claimed. The process disclosed therein is fundamentally limited in terms of productivity and structural control. As disclosed therein, nanowires that are simply entrained in the bulk gas flow during growth. As such, nanowires can freely interact with each other, particularly as their volumetric density increases, resulting in a number of undesirable outcomes. For example, Van der Waals forces would drive nanowires to irreversibly agglomerate and ultimately precipitate. This behavior reduces the maximum allowable nanowire density inside the reactor and consequently reduces reactor productivity. Further, liquid catalyst nanoparticles would likely deform upon nanowire-nanowire collision. Catalyst nanoparticle perturbations can result in undesirable nanowire structures (e.g., kinks) and even the termination of growth. Still further, nanowire-nanowire collisions may induce structural damage (e.g., breaking).
Chinese Patent No. 102553557B, entitled “Preparation method of hollow glass microsphere with directionally grown titania nanotubes on surface,” discloses a method to prepare a hollow glass microsphere with directionally grown titania nanotubes on the surface. Its method includes coating a layer of TiO2 film on the surface of the hollow glass microsphere by a surface sol-gel process. To obtain the hollow glass microsphere with the directionally grown quantum dot modified titanium dioxide nanotubes on its surface, processes involving calcination, hydrothermal and nanocomposite material synthesis are performed. The prepared hollow glass microsphere with the directionally grown quantum dot modified titanium dioxide nanotubes are used as a photocatalyst in treating offshore petroleum contaminants.
Existing methods for nanostructure production cannot simultaneously meet productivity, yield and functional requirements of many applications. For example, thermoelectric technologies demand Si nanowires in kilogram quantities with dopant concentrations of ˜1020 cm−3.
Based on the foregoing, there is a need for a cost-effective, versatile and reliable solution to produce nanostructures on a large scale, where individual nanostructures are configurable to a fine degree, for example, in terms of diameter, length, morphology, crystal structure, composition, and heterostructure.